Configurable background map layers for debug UI

[leonardehrenfried]

Is your feature request related to a problem? Please describe.

The debug UI has only a single background layer, the OSM base tiles. It would be nice if more can be selected from and if they could be configured per installation.

Describe the solution you'd like

• Add Positron as a map layer by default
• Make it possible to have additional layers via a new file called debug-ui-config.json.

Describe alternatives you've considered

Keeping the config inside router-config.json but we voted on Gitter to create a new file.

cc @fpurcell

[leonardehrenfried]

Here is what the debug UI looks like with the TriMet aerial photo background layer.